Answer choice B was written to try to convey the idea that women are not competing against each other. I am not sure if this is what you get when you read it, but it is really trying to say that the women do not compete with each other.
And while I certainly see your point, I would say that if the women are competing against each other the men would be as well, right? I mean, if the two women are running against each other one of them has to win right? So that is a 50% win rate and if the men are running against each other that is a 50% win rate.
Actually the stimulus indicates that men and women have the same rate of winning. Quote: "Last year in the United States, women who ran for state and national offices were about as likely to win as men."
So that is really why choice B is wrong. It focuses on women who do run not winning, when the argument says that the women win just as often as men, so the real focus "is not that women have difficulty winning elections but that so few women want to run."
Now we do want to weaken this, but not by contradicting one of the facts. What you are trying to explain is why only 15% of those running for office are women, not why women don't win (we are told that they win just as often as men do).
Just a couple of things to consider...hope it helps